This invention relates to novel, improved high quality brominated styrenic polymers eminently well suited for use as flame retardants in thermoplastic polymer compositions.
Brominated polystyrenes are well established as flame retardants for use in thermoplastics, e.g., polybutylene terephthalate, polyethylene terephthalate and nylon (a.k.a. polyamides). Recently, interest has been shown for expanding their use to syndiotactic polystyrene and polycyclohexylene dimethylene terephthalate. Generally, brominated polystyrenes are produced by a reaction between polystyrene and a brominating agent (e.g., bromine or bromine chloride) in the presence of a solvent (e.g., dichloroethane) and a Lewis acid catalyst. Heretofore the art has proffered many processes which are claimed to produce a superior brominated polystyrene. See, for example, U.S. Pat. Nos. 4,200,703; 4,352,909; 4,975,496 and 5,532,322.
Despite these efforts, previously-known brominated polystyrene flame retardants remain deficient in certain properties which translate into deficient performance of thermoplastic polymer blends in which they are used when the blends are subjected to polymer processing conditions.
To better understand some of the reasons for these deficiencies, it is helpful to consider some of the structural characteristics of previously known brominated polystyrenes. To begin with, the bromine content of a brominated polystyrene is the sum of (1) the bromine which is substituted onto the aromatic portions of the polymer, (2) the bromine which is substituted onto the aliphatic portion(s) of the polymer, e.g., the polymer backbone or alkyl substitution which is present due to alkylation of the aromatic portion of the polymer, and (3) any ionic bromine present, e.g., sodium bromide. The alkylation of aromatic rings in the brominated polystyrene is catalyzed by the Lewis acid catalyst used in producing the brominated styrenic polymer, and the reaction solvent (usually a 1-3 carbon atom dihaloalkane) serves as the alkylating agent. The bromine for (1) is referred to herein as aromatic bromide, while the bromine for (2) is referred to as aliphatic bromide. Even though ionic bromine can contribute to the total bromine content, its contribution to the total bromine content is small. Nevertheless, as pointed out in U.S. Pat. No. 5,328,983, ionic impurities in brominated polystyrene may degrade polymer formulations in respect to their ultimate electrical properties, and also may result in corrosion of processing equipment or in the corrosion of metallic parts in their end-use applications.
Aliphatic halide content of brominated polystyrenes is not desirable as aliphatic halide is not as thermally stable as aromatic halide and, thus, aliphatic halide can be easily converted to hydrogen halide, e.g., HBr or HCl, under normal end-use processing conditions. Aliphatic bromide and chloride are generally referred to by the art and quantified, respectively, as hydrolyzable bromide and hydrolyzable chloride since such halides are easily hydrolyzed as compared to aromatic halides.
Corrosion of metal processing equipment such as melt blenders, extruders, and molding machines, attributable to the release of hydrogen halide from prior brominated polystyrene flame retardants when molding thermoplastic polymers containing such flame retardants under thermal processing conditions is thus one deficiency of such flame retardants. In the presence of moisture, hydrogen chloride and hydrogen bromide released from the brominated polystyrene in the blend during exposure to the elevated polymer processing temperatures can result in acid formation and consequent metal corrosion.
To evaluate brominated styrenic polymers for their tendencies to release hydrogen halide under thermal processing conditions, use is made of the method described in U.S. Pat. No. 5,726,252 and referred to therein as the Thermal Stability Test Procedure. In essence, this method indicates the content of halogen atoms in the brominated polystyrene that is not bonded directly to the aromatic rings and thus is more readily released from the polymer when at elevated temperature. The Thermal Stability Test is described in greater detail hereinafter.
Prior brominated polystyrenes have oftentimes been deficient in their color characteristics. The manufacturer of molded thermoplastic articles generally finds it of advantage to have available a flame retardant which will not contribute excessive color to the molded products or otherwise interfere with color matching specifications applicable to a given product. Thus in general, the lower the color of the brominated styrenic polymer (i.e., the whiter the flame retardant), the better.
For many applications it is desirable to have a flame retardant which produces polymer blends having good flow characteristics. Good flow characteristics can significantly reduce processing time in the manufacture of molded thermoplastic products. In addition, good molten polymer flow tends to ensure that the molds are completely filled, thereby resulting in products with better surface characteristics and appearance.
The chlorine content of brominated polystyrenes is credited to chlorine which, like the bromine, is chiefly part of the polymer structure as an aromatic and/or an alkyl chloride. The use of bromine chloride as the brominating agent is the largest contributor to the chlorine content. However, chlorinated solvents and/or chlorine-containing catalysts used in the production of the brominated polystyrene may also contribute to the chlorine content of the brominated polystyrene. Apart from whether the halide is present as an aromatic or aliphatic halide, it is also desirable to minimize the total chlorine content of the brominated polystyrene as chlorine is not as efficacious or as stable a flame retardant constituent as is bromine.
Prior art brominated polystyrene polymers that have been evaluated for thermal stability have exhibited a 1% weight loss at temperatures less than 336xc2x0 C. when submitted to Thermogravimetric Analysis (TGA) and, indeed, most have exhibited a 1% weight loss at temperatures around 300xc2x0 C. Such low TGA 1% weight loss thermal stability temperatures, while acceptable in actual practice, are much less desirable than TGA 1% weight loss thermal stability temperatures of 340xc2x0 C. or above, especially under the high temperatures to which flame retarded thermoplastics formulated with such brominated polystyrene polymers are exposed during processing.
It would be especially desirable if most if not all of the above-mentioned disadvantages of brominated polystyrenes could be avoided or at least minimized. For example, it would be of considerable advantage if a more thermally stable brominated styrenic polymer, e.g., brominated polystyrene, could be provided that releases minimal amounts of hydrogen halide under thermal processing conditions, thereby greatly reducing the opportunity for corrosion of processing equipment to occur. It would be especially desirable and of considerable advantage, if a brominated styrenic polymer, e.g., brominated polystyrene, could be provided that has both improved melt flow characteristics and improved thermal stability. It would also be of great advantage if these improvements could be achieved without material sacrifice of other important properties, and if possible with concomitant provision of other desirable properties such as low ionic bromide content, minimal (if any) chlorine content, and/or less odor properties due to solvent residues. It would also be very desirable and advantageous if, in addition to high thermal stability in the Thermal Stability test, a brominated styrenic polymer, e.g., brominated polystyrene, could exhibit low color-forming tendencies, and improved melt flow properties. It would also be of considerable advantage if a brominated styrenic polymer, e.g., brominated polystyrene, could be provided that, even though it has a total bromine content of 60 wt % or more, and indeed of 67 wt % or more, possesses the combination of improved melt flow properties and low color-forming tendencies plus one or more other desirable properties. It would be of even further advantage if each or all of the foregoing advantages could be achieved with brominated styrenic polymers, e.g., brominated polystyrenes, that have very low chlorine contents. Also, it would be highly advantageous if a brominated styrenic polymer, e.g., brominated polystyrene, could be provided that possesses the combination of a total bromine content of 50 wt % or 60 wt % or more, improved melt flow properties, low color-forming tendencies, and a TGA temperature for a 1% weight loss, when submitted to thermogravimetric analysis under nitrogen, of 340xc2x0 C. or above. And it would be even more advantageous if, in addition, this combination could be achieved with a brominated styrenic polymer, e.g., brominated polystyrene, that has a thermal stability in the Thermal Stability test of 200 ppm or less, preferably 150 ppm or less, and especially 100 ppm or less.
This invention is deemed to enable the achievement of most, if not all, of the above-mentioned advantages in a highly satisfactory manner.
In accordance with one embodiment of this invention, new, high quality brominated styrenic polymers are provided that release minimal amounts of hydrogen halide under thermal processing conditions, thereby greatly reducing the opportunity for corrosion of processing equipment to occur during polymer processing operations. In addition, these new brominated styrenic polymers possess or exhibit other superior properties or characteristics enhancing their utility as flame retardants for thermoplastic polymers of various types, including for example high-impact polystyrenes, glass-filled polyesters and glass-filled nylons.
Thus one embodiment of this invention is a brominated styrenic polymer having (i) a total bromine content above about 50 wt % (preferably above about 60 wt %, and more preferably at least about 67 wt %), and (ii) thermal stability in the Thermal Stability Test described hereinafter of 200 ppm HBr or less, preferably 150 ppm of HBr or less, and more preferably 100 ppm of HBr or less. Such brominated styrenic polymer is especially suitable for use as a flame retardant when blended in at least a flame retardant amount with a thermoplastic polymer, preferably a thermoplastic engineering thermoplastic polymer. Particularly preferred in this embodiment is a brominated styrenic polymer which is further characterized in that it has, prior to blending, a chlorine content, if any, of less than about 700 ppm Cl (more preferably, less than about 500 ppm Cl, and still more preferably less than about 100 ppm Cl).
Another embodiment is a brominated styrenic polymer having (i) a total bromine content above about 50 wt % (preferably above about 60 wt %, and more preferably at least about 67 wt%), (ii) a TGA temperature for 1% weight loss which is 340xc2x0 C. or higher, preferably within the range of from about 340xc2x0 C. to about 380xc2x0 C., and more preferably within the range of from about 345xc2x0 C. to about 380xc2x0 C., and (iii) thermal stability in the Thermal Stability Test described hereinafter of 200 ppm HBr or less, preferably 150 ppm of HBr or less, and more preferably 100 ppm of HBr or less. Such brominated styrenic polymer is especially suitable for use as a flame retardant when blended in at least a flame retardant amount with a thermoplastic polymer, preferably a thermoplastic engineering thermoplastic polymer. Particularly preferred in this embodiment is a brominated styrenic polymer which is further characterized in that it has a chlorine content, if any, of less than about 700 ppm Cl (more preferably, less than about 500, and still more preferably less than about 100 ppm Cl).
The above brominated styrenic polymers of this invention are preferably a brominated polystyrene.
Pursuant to a preferred embodiment of this invention there is provided a brominated anionic styrenic polymer, preferably a brominated anionic polystyrene, that has (i) a total bromine content above about 50 wt %, preferably at least about 60 wt %, more preferably at least about 67 wt %, and still more preferably at least about 68 wt%; (ii) a thermal stability in the Thermal Stability Test of 200 ppm of HBr or less, preferably 150 ppm of HBr or less, and more preferably 100 ppm of HBr or less; and (iii) a xcex94E color value, measured using 10 wt % solutions in chlorobenzene, of less than about 10, preferably less than about 7. and more preferably less than about 5. More preferred brominated anionic styrenic polymers of this embodiment such as brominated anionic polystyrenes have GPC weight average molecular weights in the range of about 5000 to about 30,000, preferably in the range of about 7000 to about 25,000, and more preferably in the range of about 8000 to about 20,000.
Another preferred embodiment of this invention is a brominated anionic styrenic polymer, preferably a brominated anionic polystyrene, that has (i) a total bromine content above about 50 wt %, preferably at least about 60 wt %, more preferably at least about 67 wt %, and still more preferably at least about 68 wt %; (ii) a thermal stability in the Thermal Stability Test of 200 ppm HBr or less, preferably 150 ppm of HBr or less, and more preferably 100 ppm HBr or less; and (iii) a melt flow index of at least about 20 g/10min, preferably at least about 25 g/10 min, and more preferably at least about 30 g/10 min in the Melt Flow Index Test as described hereinafter. More preferred brominated anionic styrenic polymers of this embodiment such as brominated anionic polystyrenes have a GPC weight average molecular weight in the range of about 5000 to about 30,000, preferably in the range of about 7000 to about 25,000, and more preferably in the range of about 8000 to about 20,000.
Still another preferred embodiment of this invention is a brominated anionic styrenic polymer, preferably a brominated anionic polystyrene, that has (i) a total bromine content above about 50 wt %, preferably at least about 60 wt %, more preferably at least about 67 wt %, and still more preferably at least about 68 wt %; and (ii) a melt flow index of at least about 20 g/10 min, preferably at least about 25 g/10 min, and more preferably at least about 30g/10 min in the Melt Flow Index Test as described hereinafter. In more preferred embodiments, these brominated styrenic polymers such as brominated polystyrene also have (I) a xcex94E color value, measured using 10 wt % solutions in chlorobenzene, of less than about 10, preferably less than about 7, and more preferably less than about 5; or (II) a TGA temperature for a 1% weight loss under nitrogen of at least about 340xc2x0 C., and preferably at least about 350xc2x0 C.; or (III) a GPC weight average molecular weight in the range of about 5000 to about 30,000, preferably in the range of about 7000 to about 25,000, and more preferably in the range of about 8000 to about 20,000. Still more preferred compositions of this embodiment possess at least any two of (I), (II), and (III), e.g., (I) and (II); (I) and (III); or (II) and (III). Even more preferred are compositions of this embodiment that possess all three of (I), (II), and (III).
A further preferred embodiment of this invention is a brominated anionic styrenic polymer, preferably a brominated anionic polystyrene, that has (i) a total bromine content above about 50 wt %, preferably at least about 60 wt %, more preferably at least about 67 wt %, and still more preferably at least about 68 wt %; (ii) a xcex94E color value, measured using 10 wt% solutions in chlorobenzene, of less than about 10, preferably less than about 7, and more preferably less than about 5; and (iii) a GPC weight average molecular weight in the range of about 5000 to about 30,000, preferably in the range of about 7000 to about 25,000, and more preferably in the range of about 8000 to about 20,000. In more preferred embodiments, these brominated styrenic polymers such as brominated polystyrene also have a TGA temperature for a 1% weight loss under nitrogen of at least about 340xc2x0 C., and preferably at least about 350xc2x0 C.
Still another preferred embodiment of this invention is a brominated anionic styrenic polymer, preferably a brominated anionic polystyrene, that has (i) a total bromine content above about 50 wt %, preferably at least about 60 wt %, more preferably at least about 67 wt %, and still more preferably at least about 68 wt %; (ii) a thermal stability in the Thermal Stability Test of 200 ppm HBr or less, preferably 150 ppm of HBr or less, and more preferably 100 ppm HBr or less; (iii) a melt flow index of at least about 20 g/10 min, preferably at least about 25 g/10 min, and more preferably at least about 30 g/10 min in the Melt Flow Index Test as described hereinafter; and (iv) a xcex94E color value, measured using 10 wt % solutions in chlorobenzene, of less than about 10, preferably less than about 7, and more preferably less than about 5. In more preferred embodiments, the brominated styrenic polymers such as brominated polystyrene described in this paragraph are further characterized in that they have a GPC weight average molecular weight in the range of about 5000 to about 30,000, preferably in the range of about 7000 to about 25,000, and more preferably in the range of about 8000 to about 20,000; and/or a TGA temperature for a 1% weight loss under nitrogen of at least about 340xc2x0 C., and preferably at least about 350xc2x0 C.
A further preferred embodiment of this invention is a brominated anionic styrenic polymer, preferably a brominated anionic polystyrene, that has (i) a total bromine content above about 50 wt %, preferably at least about 60 wt %, more preferably at least about 67 wt %, and still more preferably at least about 68 wt %; (ii) a thermal stability in the Thermal Stability Test of 200 ppm HBr or less, preferably 150 ppm HBr or less, and more preferably 100 ppm HBr or less; and (I) that is essentially free of impurities selected from the group consisting of (a) methylene chloride, (b) ethylene dichloride, and especially (c) bromodichloroethane, (d) dibromochloroethane, (e) dibromodichloroethane, (f) tribromochloroethane, and (g) any mixture of two or more of the foregoing, especially a mixture that contains at least one of (c) through (f); (II) that has an ionic bromine content of 2000 ppm or less, preferably 1500 ppm or less, more preferably 1000 ppm or less, and still more preferably 500 ppm or less; or (III) that contains, if any, less than about 700 ppm Cl, preferably less than 500 ppm Cl, and more preferably less than 100 ppm Cl. More preferred compositions of this embodiment possess at least any two of (I), (II), and (III), e.g., (I) and (II); (I) and (III); or (II) and (III). Still more preferred are compositions of this embodiment that possess all three of (I), (II), and (III).
In prior commonly-owned copending application Ser. Nos. 09/151,193; 09/151,194; and 09/170,487, it is indicated to be preferable, or even desirable because of cost and lack of ready availability, not to employ anionically-produced polystyrene in forming brominated polystyrenes. Nevertheless brominated polystyrenes made from anionic polystyrenes pursuant to this invention possess a combination of excellent properties. Such properties include, for example, very high thermal stability in the Thermal Stability test, very low xcex94E color value, and high melt flow even with bromine contents of 67 wt % or more. Moreover, tests conducted using brominated polystyrenes produced from anionic polystyrenes by other entities known for their expertise in the field of brominated polystyrenes showed that those products were substantially inferior in properties.
Other embodiments and features of this invention will be further apparent from the ensuing description and appended claims.